1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method therefor, and more particularly to a semiconductor device including a high dielectric constant insulating film and a manufacturing method therefor.
2. Background Art
In recent years, the integration density of semiconductor integrated circuit devices has considerably increased. As such, devices such as transistors for MOS (Metal Oxide Semiconductor) devices, for example, have been miniaturized and enhanced in performance. Especially, the gate insulating films, which are a component of the MOS structure, have become thinner and thinner to accommodate the miniaturization, higher-speed operation, and lower-voltage operation of the transistors.
Conventionally, silicon oxide films and silicon oxynitride films have been used for gate insulating films. However, reducing the film thickness of gate insulating films of such materials (considerably) increases the leakage current.
On the other hand, CMOS (Complementary Metal Oxide Semiconductor) devices of a sub-0.1 μm generation must employ gate insulating films having an equivalent oxide thickness of 1.5 nm or less. Therefore, it is proposed that metal oxide films or metal silicate films, which have a high relative permittivity, may be used as the gate insulating films to provide an increased film thickness and thereby reduce the leakage current.
However, when a metal oxide film or a metal silicate film is directly formed on a silicon substrate, the surface of the silicon substrate is oxidized due to heat treatment during or after the film forming process, resulting in formation of a silicon oxide film whose film thickness is as large as 1.5 nm or more. Therefore, it has been difficult to obtain an equivalent oxide thickness of 1.5 nm or less. Furthermore, there is another problem with this type of silicon oxide film. That is, the silicon in a univalent, bivalent, or trivalent state existing in the silicon oxide film bonds to oxygen, forming suboxides, which contain more silicon than the stoichiometric composition of silicon oxide, or silicon dioxide (SiO2). This leads to an increased leakage current. To overcome: this problem, a method is proposed for forming a silicon oxide film, a silicon nitride film, or a silicon oxynitride film on the silicon substrate as the underlying film before forming a metal oxide film or a metal silicate film (see, for example, Japanese Laid-Open Patent Publication No. 11-126902 (1999)).
When the silicon oxide film formed on the surface of the silicon substrate contains only a small amount of suboxide, the subsequent metal oxide film or metal silicate film forming process and the heat treatment process following it cause only a small increase in the film thickness of the silicon oxide film, providing relatively good leakage characteristics. With a laminated structure made up of a high dielectric constant insulating film and a silicon oxide film, however, it is necessary to control the film thickness of the silicon oxide film such that it is 1.0 nm or less, thereby the suboxides existing in the surface portion of the silicon substrate contribute more to an increase in the leakage current.
On the other hand, it is proposed that processes such as removal of the natural oxide film on the surface of the silicon substrate, formation of films, and monitoring of the formed films may be carried out in succession within a cluster tool under reduced pressure or filled with dry nitrogen (see, for example, Japanese Laid-Open Patent Publication No. 2002-270596). This arrangement is said to prevent a absorption of contaminants and water due to exposure to the atmosphere. According to the above Japanese Laid-Open Patent Publication No. 2002-270596, a silicon substrate is introduced into a precleaning chamber under reduced pressure in which the substrate is treated with dilute hydrofluoric acid. The silicon substrate is then washed with water and dried before it is transferred to a gate oxidation furnace after a re-evacuation process. However, since the moisture within the processing chamber is difficult to control, it is considered that it is difficult to produce a semiconductor device including a metal oxide film or a metal silicate film by the above method.
Further, conventionally, silicon nitride films, which have a higher relative permittivity than silicon oxide films, have been used as metal oxide film capacitors having a high dielectric constant. Recently, however, silicon oxynitride films are used as gate insulating films since forming a silicon nitride film in contact with a silicon substrate may increase the interfacial level (see, for example, Japanese Laid-Open Patent Publication No. 2-256274 (1980)).
When the surface of a silicon substrate is directly nitrided, first the natural oxide film existing in the surface of the silicon substrate and containing a large amount of suboxide is removed with an aqueous solution of dilute hydrofluoric acid. Then, a hydrogen termination process is carried out to prevent the surface of the silicon substrate from being oxidized again, before the surface is nitrided (see, for example, Japanese Laid-Open Patent Publication No. 2002-324902). However, it is difficult to hydrogen-terminate all silicon atoms present in the surface of the silicon substrate. Therefore, water and oxygen are absorbed to silicon atoms not terminated with hydrogen in the water washing/drying process after the dilute hydrofluoric acid treatment or in the transfer process to the nitriding equipment, forming suboxides of silicon. Because of this, the silicon nitride film, which has a film thickness of approximately 1 nm, has an oxygen concentration of 0.5 atom % or more.
Other documents also describe forming a silicon oxynitride film at the interface between the silicon substrate and the metal oxide film having a high dielectric constant (see, for example, Japanese Laid-Open Patent Publications Nos. 2001-257344, 2002-305196, and 2002-324901). These methods first form a silicon oxide film and then nitride the formed silicon oxide film in order to reduce the nitrogen concentration at the interface between the silicon substrate and the (resultant) silicon oxynitride film and thereby reduce the interfacial level. However, since they form one to three atomic layers of silicon oxide (film) and nitride them, the same problem as that described above in connection with formation of a silicon oxide film arises.
Furthermore, even if a high quality silicon oxide film, silicon nitride film, or silicon oxynitride film is used as the underlying film, an increase in the film thickness of the underlying film occurs when a high dielectric constant insulating film is formed thereon. Therefore, the film forming method must be capable of reducing the increase in the film thickness of the underlying film. When one of the above films is used as the underlying film, the high dielectric constant insulating film is preferably formed by a CVD (Chemical Vapor Deposition) technique rather than a sputtering technique, which inflicts larger damage to the underlying film, or other deposition techniques, which provide poorer surface uniformity. However, the CVD technique uses an oxidizing gas such as water molecules, oxygen molecules, oxygen radicals, or ozone, making it difficult to reduce the increase in the film thickness of the underlying film.